Organic Chemistry Lecture 10 PDF

ORGANIC CHEMISTRY lecture 10 1/7 Stereochemistry and stereoisomer Stereochemistry : is the science that deals with structure in three dimensions. Isomers : are different compounds that have the s...

ORGANIC CHEMISTRY lecture 10 1/7 Stereochemistry and stereoisomer Stereochemistry : is the science that deals with structure in three dimensions. Isomers : are different compounds that have the same molecular formula.which are called structural isomers. Steroisomer : is the particular kind of isomers that are different from each other only is the way of orientation of atoms is space. Enantiomers : is the mirror image isomers. These isomers have the same physical properties except the rotation of the plane polarized light. They are differ only in the way of orientation of the atoms in space.They are not superimposable to each other. The non superimposability of mirror image leads to enantiomer and also gives each of them their optical activity. (enantio =opposite ) for example , biochemical compound for leucine Chiral compound :- The structural isomers have different physical properties in M.P , b.p. density If the isomers have identical properties it must have identical molecular structure. It was discovered big number of compounds which contradict this fact Consider the compound leucine C6H13O2N. this compound present as two isomers H3C CH3 CH CH2 H C CO 2H Chiral atom NH2 These isomers are differ in taste , the first has bitter taste and the other has sweat taste because they are different isomers. because they are differ in orientation of atoms in space NH2 CO2H CO2H NH2 H C H C CH2CH (CH3)2 CH2CH (CH3)2 Two compounds:-three dimensional structure of leucine Stereoisomer are related in an important way that they are mirror-image of each other. Further more these mirror-image are not superimposed 1  ORGANIC CHEMISTRY lecture 10 2/7 NH2 NH2 CO2H HO 2C H C C H CH2CH (CH3)2 (CH3)2CHCH2 Leucine Mirror-image relationship of the two stereoisomers of lecine.We them enantiomers Remember that all objects have a mirror image. However objects can be superimposed on their mirror image How we distinguish between chiral and achiral compounds First we must determined whether this isomers superimposed or not Chiral compound :- any compound which has one or more carbon atoms bonded to four different atoms or groups Cl NH2 H3C * * * CH CH CH3 CH3 CH2 CH CH3 CH CO2H H3C Cl The chiral carbon atom is un symmetrical atom. The different between chirality and optical activity : chirality : objects which are mirror images but not superimposed , optical activity is a physical property in which the compound which rotate the plane polarized light. If the compound composed from two enantiomers i.e. one of them (-) and the other is (+) the net rotation = zero A mixture of equal amount of two enantiomers is called racemic mixture , it is optically inactive Example The light-catalyzed butane and chlorination of to form 2-chlorobutane as one the product. CH2CH3 CH2CH3 light H H CH3 CH2 CH2 CH3 + Cl2 C Cl + Cl C CH3 CH3 2-chloro butane contain chiral carbon but it is inactive because equal amounts of dextrorotatory and levorotatory isomers are formed in equal amount. 2  ORGANIC CHEMISTRY lecture 10 3/7 In living system The reaction will produce only one of the enantiomers ,the compound will be optically active and this is the main different between reaction occurs in lab. and reaction occurs in living system. Naming of stereoisomers we must mention the specific rotation of conferbration Diasteroisomers :- The compound which contain two or more chiral centers can exist as more than two sreroisomers. They are not mirror images and not superimposable. General rule :- the maximal number of stereoisomers is 2n n= number of chiral atoms * * CH3 CH CH CH CH3 OH OH OH 2 chirals example of compound containing two chiral centers is the carbohydrate 2,3,4-trihydroxybutyraldehyde * * HOCH2 CH CH CHO OH OH It has four stereoisomers CHO CHO HO C H H C OH HO C H H C OH CH2OH CH2OH I II L-erythrose D-erythrose They are pair of enantiomers called Erythose CHO CHO H C OH HO C H HO C H H C OH CH2OH CH2OH III IV L-thereose D-thereose 3  ORGANIC CHEMISTRY lecture 10 4/7 These pair of enantiomers called Threose I & II are mirror image and they are not superimposed III & IV are mirror image and they are not superimposed ( Ι , ΙΙΙ ), ( ΙΙ , Ιv) the compounds are not mirror image (Diastereomers ) and not superimposable Diastereomers have different physical properties Ι is liquid Ιv is solid Solubility Erythrose highly soluble in ethanol while Threose slightly soluble in ethanol Meso stereoisomers :- Certain molecules that contain two chiral atoms do not exist as four stereoisomers. Specific example is Tartaric acid OH * HO2C CH CH CO2H * OH CO2H CO2H CO2H CO2H H C OH HO C H H C OH OH H C OH HO C H H C OH H C OH H OH C OH CO2H CO2H CO2H CO2H pair of enantiomers identical compound they are called mesoisomers CO2H CO2H H C OH rotation 180 HO C H H C OH HO C H CO2H CO2H III IV Mesoisomers is optically inactive because they have plane of symmetry. it superimposed on its mirror image 4  ORGANIC CHEMISTRY lecture 10 5/7 Resolution of Racemic mixture Resolution is process of separating of enantiomers into pure stereoisomer 1- mechanical separation :- it was performed by Pasteur when he crystallize tartaric acid salt he found two types of crystals : a- crystals which are dextrorolotary b- crystals which are levorototary any compound has plane of symmetry is optical inactive he has able to separate them (by using a pair of tweezers and magniting lens ) between right handed crystals and left handed crystals 2- Enzymetic separation :- this separation can be done in living system , The living system react with one of the enantiomers and leaving the other. Pasture found that a yeast in urine reacted with (+) tartaric acid when he gave the yeast racemic tartaric acid he found that only the dextrorotatory isomer was consumed , The levorotatory isomers was isolated at the end of the reaction 3- Separating diastereomers diastereomers have different physical properties , This fact provides us a method of resolving a racemic mixture , by reacting of a racemic mixture with an optically active reagent Specification of configuration: R and S Now, a further problem arises. How can we specify a particular configuration in some simpler, more convenient way than by always having to draw its picture? The most generally useful way yet suggested is the use of the prefixes R and S. According to a procedure proposed by R. S. Cahn (The Chemical Society, London), Step 1. Following a set of sequence rules, we assign a sequence of priority to the four atoms or groups of atoms attached to the chiral center. In the case of CHClBrI, for example, the four atoms attached to the chiral center are all different and priority depends simply on atomic number, the atom of higher number having higher priority. Thus I, Br, CI, H. 5  ORGANIC CHEMISTRY lecture 10 6/7 Step 2. We visualize the molecule oriented so that the group of lowest priority is directed away from us, and observe the arrangement of the remaining groups. If, in proceeding from the group of highest priority to the group of second priority and thence to the third, our eye travels in a clockwise direction, the configuration is specified R (Latin : rectus, right); if counterclockwise, the configuration is specified S (Latin: sinister, left). Thus, configurations I and II are viewed like this: R S and are specified R and S, respectively. A complete name for an optically active compound reveals if they are known both configuration and direction of rotation, as, for example, (S)-(+)-sec- butyl chloride. A raccmic modification can be specified by the prefix RS, as, for example, (RS)-sec-butyl chloride. We must not, of course, confuse the direction of optical rotation of a compound a physical property of a real substance, like melting point or boiling point with the direction in which our eve happens to travel when we imagine a molecule held in an arbitrary manner. So far as we are concerned, unless we happen to know what has been established experimentally for a specific compound, we have no idea whether (+ ) or ( - )rotation is associated \\ith the (R)- or the (S)-configuration. 6 ORGANIC CHEMISTRY lecture 10 7/7 S R 7